The present invention relates generally to an apparatus for driving an elevator and, in particular, to an apparatus for reducing the power required to drive an hydraulic elevator.
An hydraulic elevator, in which an hydraulic drive of the piston-cylinder type is positioned beside the elevator car in the elevator shaft, is shown in the German patent document DE GM 69 26 58. The drive cylinder is mounted on a bracket in the shaft pit and a cable guide roller is mounted below the bracket. Two additional cable guide rollers are attached at the upper end of the piston rod of the drive. One end of a hoist cable is fixed at the shaft wall in the region of the bracket and the other end is fixed in the upper region of the elevator shaft. The hoist cable extends from the lower fixed point upwardly and over one of guide rollers at the upper end of the piston rod, downwardly and about the guide roller at the bracket, upwardly and about the other guide roller at the upper end of the piston rod, downwardly and about two guide rollers attached to the base of the elevator car, and upwardly to the upper fixed point in the shaft. Thus, the elevator car is suspended in a downwardly directed loop of the hoist cable. Due to the looping of the hoist cable, the elevator car moves at twice the piston speed and twice the vertical travel of the piston rod. Without considering the friction losses, the force required to be generated at the piston rod corresponds to twice the value of the weight of the elevator car and the conveyed load.
A method and apparatus for improving the performance of a motor-driven hydraulic elevator, in which the speed of travel of the elevator car is controlled through change in the rotational speed of an electrical motor driving an hydraulic pump, is shown in the German patent document DE-OS 38 36 212 (see U.S. Pat. Nos. 5,014,823 and U.S. Pat. No. 5,048,644). An improvement in the performance and a reduction in the thermal loading of the electrical motor is achieved by reducing the oil pressure in the main hydraulic line to a predetermined constant level with the aid of a reducing valve during the drive of the elevator in the downward direction. The reducing valve is provided with a return path in which a pressure-equalizing valve controls the flow volume through the reducing valve to the main line. Thus, a smaller quantity of electrical energy is needed during the downward travel, whereby the thermal loading of the electrical motor is also reduced.